PTFE is plastics having excellent heat and chemical resistance, and porous membranes made of PTFE are extensively utilized as filter media for corrosive gases and liquids, permeable membranes for electrolysis, and battery separators. Their use as a filter medium in the precision filtration of various gases and liquids used in the semiconductor industry has become an extremely important application thereof.
In order that a porous membrane be an excellent filter medium, the pore diameter distribution of the membrane should be sharp and, when a fluid is allowed to permeate through the membrane pores at a certain pressure, the amount of the fluid passing through the membrane per unit time should be large. It has conventionally been known that the smaller the membrane thickness, the higher the permeation rate for a fluid, when porosity and pore diameter are constant. However, porous membranes having smaller membrane thicknesses may suffer deformation due to pressure exerted thereon during filtration and, as a result, the pore diameters are changed or, in some cases, the membranes are broken to be unable to function as a filter medium. In addition, the handling properties of such thin porous membranes are so poor that they are apt to be damaged when processed into filter modules or fixed to filter holders.
For the purpose of eliminating these problems, several multilayer PTFE porous membranes have been proposed which comprise a filter layer having small pore diameters and a support layer having larger pore diameters than the filter layer. Conventional processes for producing such membranes include, for example, (1) a process in which one or more PTFE porous structures having smaller pore diameters and one or more PTFE porous structures having larger pore diameters are superposed on each other in an unsintered state and then press-bonded, and the resulting film is sintered at a temperature not lower than the melting point of PTFE to obtain a multilayer PTFE porous membrane (as described in JP-A-54-97686), and (2) a process in which an unsintered film is stretched between a roll revolving at a low speed and a roll revolving at a high speed, while a temperature gradient is being created in the direction of the thickness of the thin film and, at the same time, a compressive force is being applied in that direction, thereby to obtain a porous membrane in which its obverse side and reverse side have different pore diameters (as described in JP-B-63-48562). (The term "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.)
Further, although intended for producing a filter medium not for precision filtration but for the separation and enrichment of mixed isotopic gases, a conventional method for manufacturing a microporous permeable membrane include (3) a process in which one or more PTFE thin films in which a liquid pore-forming agent has been incorporated and one or more other PTFE thin films in which a liquid pore-forming agent has been incorporated are superposed on each other, the resulting assemblage is rolled to bond the thin films with each other, and then the liquid pore-forming agents are extracted with a low molecular weight liquid to form pores, thereby obtaining a multilayer PTFE porous membrane comprising at least two layers having different average pore diameters (as described in JP-B-55-22504).
In process (1) above, sintering of unsintered stretched superposed films at a temperature not lower than the melting point of the PTFE powders gives a fusion-bonded united film, as disclosed in JP-A-51-30277. When unsintered sheets or films made from PTFE fine powders are lapped and then sintered, the respective layers are fusion-bonded with each other to give a united shape, and this technique has conventionally been known as, for example, a manufacturing method for PTFE-lapped electrical cables and PTFE-lapped tubes or pipes. Therefore, the method of superposing stretched porous structures with different pore diameters on each other and sintering the assemblage at a temperature not lower than the melting point of the PTFE has been quite common in the art. Process (1) above is disadvantageous in that it necessitates a step of separately forming two or more sheets or films having different porosities and the subsequent sintering step, which should be performed while the sheets or films superposed on each other are being pressed together. Furthermore, in order to industrially produce films with extremely small thicknesses or low strengths by such a laminating technique, expensive facilities and a high degree of skill are required so as to avoid occurrence of wrinkling, breakage, etc. in the process.
Process (2) above is disadvantageous in that the stretching, which is conducted between rolls, is limited to monoaxial stretching and biaxial stretching cannot be used in this method.
Process (3) above is characterized in that a membrane comprising two or more layers having different average pore diameters is obtained not through stretching, but by varying the packing densities of emulsion-polymerized PTFE powders having different primary particle sizes and shapes and also by use of pore-forming agents of different kinds. However, it should be noted that the pores in this membrane are mere spaces among emulsion-polymerized PTFE particles, that is, the unsintered film obtained from emulsion-polymerized PTFE by a paste-processing technique has a structure which nearly is the closest packing of the PTFE primary particles. Illustratively state, the primary particles have specific gravities of from 2.1 to 2.3 and the processed film has a bulk specific gravity of from 1.5 to 1.6 in the case where an ordinary petroleum solvent or the like has been used for shaping the film, and the difference between the specific gravities is ascribable to pores, which are spaces among the polymer particles. Such a membrane has a poor filter performance, i.e., very poor fluid permeability, and also has a very low strength compared with sintered membranes. If the unsintered multilayer membrane is sintered in order to increase its strength, it becomes non-porous to be unusable as a filter medium for fluids in the semiconductor industry.
It has been proposed to obtain a multilayer porous membrane by a method in which rolled PTFE sheets containing a lubricant are superposed on each other, and the resulting assemblage is further rolled to a smaller thickness and then stretched (as described in JP-A-57-131236). The porous membrane obtained by this process, however, consists of layers that do not differ in porosity from each other at all, although it has high inter-layer bonding strength. JP-B-56-17216 discloses a process for producing a single-layer PTFE porous membrane having a high tensile strength. Conventionally, the size of small pores has been controlled by stretching and amorphous-lock, especially by changing the temperature, the drawing rate per unit time, and the draw ratio.
On the other hand, unsymmetrical membranes consisting of an extremely thin filter layer and a support layer which is thicker and has larger pore diameters than the filter layer are manufactured from cellulose acetate or polysulfone. However, since such unsymmetrical membranes are obtained by wet coagulation processes, the membrane material is required to be soluble in the solvent used and, hence, this method has not been applicable to PTFE, which is not soluble in any ordinary solvent at all.